Add rate-limit observability events

[Eministar]

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• I have checked the PRs for upcoming features/bug fixes.
• I have read the contributing guidelines.
• I applied the code formatter to my changes with ./gradlew format

Changes

• Internal code
• Library interface (affecting end-user code)
• Documentation
• Other: _____

Closes Issue: NaN

Description

This PR adds a structured observability hook for JDA rate-limit processing, focused on the default SequentialRestRateLimiter.

What was added

1. Public event model for rate-limit telemetry

• Added RestRateLimiter.RateLimitEvent with typed event categories:
  • REQUEST_QUEUED
  • REQUEST_REQUEUED
  • BUCKET_UPDATED
  • RATE_LIMITED
  • BACKOFF
• Added event metadata for route/bucket/queue state/retry timing/scope.

2. Configurable event consumer in RestConfig

• Added setRateLimitEventConsumer(...)
• Added getRateLimitEventConsumer()

3. Wiring into runtime initialization

• JDAImpl now passes the configured event consumer into RestRateLimiter.RateLimitConfig.

4. Emission in default limiter

• SequentialRestRateLimiter now emits structured events during queueing, retries, bucket updates, 429 handling, and backoff windows.

5. Documentation

• Added Javadocs for all new public API types/methods.
• Added README section with usage example (Rate-Limit Observability).

Usage example

RestConfig restConfig = new RestConfig().setRateLimitEventConsumer(event -> {
    if (event.getType() == RestRateLimiter.RateLimitEvent.Type.RATE_LIMITED) {
        int shardId = event.getJDA().getShardInfo() == null
                ? -1
                : event.getJDA().getShardInfo().getShardId();

        System.out.printf(
                "rate-limit shard=%d route=%s bucket=%s retryAfterMs=%d scope=%s%n",
                shardId,
                event.getRoute().getBaseRoute(),
                event.getBucketId(),
                event.getRetryAfterMillis(),
                event.getScope());
    }
});

JDABuilder.createDefault(BOT_TOKEN)
    .setRestConfig(restConfig)
    .build();  

Scope

This is one logical feature: observability for REST rate-limit handling.
No unrelated refactors are included.

Validation

• Ran ./gradlew format
• Ran ./gradlew build
• Build passed successfully